THE mechanical camera for the C/Y system is the Contax S2/S2b. Elsewhere, there's a lot of choice. You could certainly make a case for one of the Leicaflex models, probably the SL. My Leicas are rangefinders, some screwmount models and an M6, but in the last few years the already pricey M system has become stupidly expensive. The M6 I bought for £800 now goes for about 3x that, and some of the lenses cost 2-4x what I paid for them. So recently, since I already have Nikon AF gear, I've bought a couple of mechanical bodies (FM & FM2n) and some manual focus lenses to go with them, building a small system that has some compatibility with the AF stuff (Nikon compatibility is complicated!), and is much easier to replace than the Leica gear if anything should happen to it. Even though Nikon is far from the cheapest manual focus option, both bodies and 6 lenses came to less than the replacement cost of one of my Leica lenses.
Some people rate one or other of the Nikon FM series as THE compact mechanical SLR (the final FM3a pulls off the neat trick of having a full range of both mechanical and electronically controlled shutter speeds), but of course there's fierce competition from the Olympus OM-1n (a real design classic) and various Pentax models. But the ultimate mechanical Nikon, and arguably THE mechanical SLR if you don't mind the size and weight, is probably the F2 in one configuration or another. Unlike the original F, you can choose a prism that will do open aperture metering with the AI lenses they've been making since the late 70s, all the way up to the AF lenses that have aperture rings. Or you can choose an old-school pre-AI style prism and have open-aperture metering with any lens that has the 'rabbit ear' coupling prongs, pre-AI or AI. Or just get a plain meterless prism. If you wanted to bet on a Nikon that can still be made to work 50 years from now if anyone is still making film, I suspect it would be this one.